Removing the screw that secures the clamping knob to the tool. The knob connector is hex-shaped. Remove the knob and turn it one or two "notches" and replace it and put the screw back. That should give you more bite on the knob.
And for those reading this thread in the future - that should be notches to the left if you are at the back end of the tool looking over the tool towards the front.